The present invention is related to improved polyanions which are particularly suitable for use with conducting polymers and especially as part of a cathode of a solid electrolytic capacitor.
Solid electrolytic capacitors are widely used throughout the electronics industry. In high voltage applications solid electrolytic capacitors with a solid electrolyte, formed by conductive polymer dispersions, give excellent high voltage performance compared to conductive polymer cathodes formed in-situ. These conductive polymer dispersions are prepared by a number of process steps including polymerization, purification, filtration, homogenization, evaporation, etc. Descriptions of these processes are provided in U.S. Pat. Nos. 5,300,575; 7,990,684; 7,270,871; 6,000,840 and 9,030,806; U.S. Patent Publication No. 2011/0049433 and PCT Publication WO 2010/089111 each of which is incorporated herein by reference.
Capacitors and methods of making capacitors are provided in U.S. Pat. Nos. 7,990,683; 7,754,276 and 7,563,290 each of which is incorporated herein by reference.
Solid electrolytic capacitors comprising conducting polymer, as the cathode, have several disadvantages. For example, solid electrolytic capacitors suffer from poor equivalent series resistance (ESR) particularly under high humidity and high temperature conditions. In addition, poor coverage of conducting polymers on corners and edges of anodized anode results in high DC leakage current. One approach for improving coverage of the corners and edges is provided in International Application WO2010089111A1, which is incorporated herein by reference, which describes a group of chemical compounds called crosslinkers or primer, which are mostly multi-cationic salts or amines. International Application WO2010089111A1 teaches the application of a solution of the crosslinker on the anodized anode prior to the application of polymer slurry to achieve good polymer coverage on corners and edges of the anodized anode. The effectiveness of the crosslinker is attributed to the cross-linking ability of multi-cationic salts or amines to the slurry/dispersion particles. While crosslinkers are advantageous for improving the coating coverage on corners and edges of the anodized anode, the addition of these crosslinkers, which are mostly ionic in nature, has the unintended consequences of degrading the humidity performance of a finished product.
Many of the problems associated with solid electrolytic capacitors have now been found to be the result of the nature of the conductive polymeric layer and particularly the polyanion counterion of the conductive polymer. The strongly acidic nature of polyanions also contributes to increased moisture absorption leading to additional problems such as increased corrosion of metals in a capacitive device. The dispersions of conductive polymer and polyanion are also typically not effective at forming an adequate coating on the dielectric which often leads to thin, or vacant, coatings thereby leading to poor leakage current. Thus, additional binders/additives in conducting polymer dispersions are required to maintain film strength during fabrication and device operation. Moreover, higher the percent solids in conductive polymer dispersion are desired to improve corner/edge coating coverage of anodized anode and possibly minimize/eliminate use of ionic cross-linker/primer.
It has been found that the use of polyanion copolymer with lower concentration of sulfonic acid groups and containing performance enhancement functional groups such as adhesion promoter/moisture retention/hydrophobic/cross-linkable groups can mitigates the above problems.
The present invention sets forth improvements in the polyanion, method of making the polyanion and conductive polymer dispersions comprising the polyanion.